The invention relates to a method for synchronizing a space telecommunications system comprising at least one satellite.
Most space telecommunications systems planned at the present time have synchronization systems the architecture of which is based on satellite/terminal synchronization offset measurements and corrections on up-links and down-links. Such architecture requires complex management on behalf of the satellite or the associated control center of the various user terminals and their synchronization offsets.
The document referenced as [1] at the end of the specification describes various functions involved in xe2x80x9coff-line orbit controlxe2x80x9d as part of a constellation of satellites, namely: determining orbit and time, calculating maneuvers in order to follow the reference orbit, carrying out the maneuvers, controlling maneuvers. The use of satellite positioning systems results in the development of a specific on-board device called xe2x80x9cnavigatorxe2x80x9d. This device can be based on two types of positioning and synchronizing systems, e.g. the DORIS and GPS systems. This document [1] describes the general hardware features of such a navigator and its interfaces with the satellite.
The document referenced as [2] describes a method and system for synchronizing a communications network, comprising several non-stationary satellites, based on transmissions of preliminary signals composed of bursts.
The object of the invention is a method for synchronizing a telecommunications spaces system comprising at least one satellite of simplified synchronization architecture.
The invention relates to a method for synchronizing a telecommunications spaces system comprising at least one satellite and several user terminals, wherein each satellite comprises: a means for transmitting synchronization information to user terminals; characterized in that each satellite further comprises:
off-line and real time means delivering tracking measurements;
a means for processing these measurements so as to smooth them through an orbit model; and in that said method comprises the following steps:
performing an accurate and real time orbit restitution measurement aboard each satellite;
transmitting this satellite orbit restitution and reference on-board time to the user terminals;
each of the user terminals, communicating with a given satellite, then synchronizing its transmissions with this satellite, with respect to satellite time.
Orbit restitution can be based on using Doppler measurements from the DORIS system or using pseudo-distance and pseudo-speed measurements from the GPS system.
Advantageously, each terminal communicating with a satellite performs the following operations:
identifying the time shift of its time reference with respect to satellite time;
theoretically calculating the terminal/satellite propagation delay from the measured orbit of the satellite and the terminal position;
transmitting the terminal/satellite up-link information telecommunications signal at the date required by the satellite, the signal being pre-compensated for Doppler-frequency and synchronization offset time.
The invention is based on the implementation of an accurate and real time orbit tracking means aboard each satellite.
Coupling the accurate on-board orbit and the synchronization architecture of the telecommunications system allows to significantly simplify the payloads of telecommunications satellites and improve the performance of communications systems.
The invention mainly applies to communications systems between a constellation of satellites and numerous user ground terminals. It can also be applied to a telecommunications service provided by one or more non-stationary satellites.
The main advantages of the invention are located at three levels of the telecommunications system: reducing satellite complexity, increasing spectral efficiency of terminal/satellite links, relieving terminal constraints.
The tasks to be performed by each satellite as part of conventional synchronization, which no longer exist in the case of navigator-controlled synchronization according to the invention, are mainly the following ones:
Measuring the synchronization offset between the satellite and each connected terminal, which requires a set of correlators (hardware).
Calculating the synchronization offset for each terminal, which requires computing power resources (software, hardware).
Managing the addresses of the various terminals connected for transferring the synchronization offsets on the down-link, which requires interconnections between the data regenerating and data switching parts at the satellite, thus making its composition more complex (software, hardware).
To this increase in satellite complexity, as part of a conventional synchronization, corresponds, for a navigator-controlled synchronization according to the invention, the mere implementation of an accurate orbit restitution device on each satellite.
The gain in spectral efficiency is expressed as follows:
n being the number of terminals connected to a satellite.
The gain in spectral efficiency is certain because, as part of navigator-controlled synchronization like according to the invention, only one down-link per satellite is required, this link being the same for all terminals.
For non-stationary satellites, knowing the accurate orbit of the satellite allows the terminal pointing constraint to be relieved.
Indeed, without such information, the terminal is pointed towards the satellite by means of theoretical ephemerides stored in memory. These ephemerides are somewhat inaccurate, which is harmful to the terminal""s pointing record (in addition to the xe2x80x9cphysicalxe2x80x9d pointing inaccuracy).
For navigator-controlled synchronization as according to the invention, the terminal receives the highly accurate and real time orbit from the satellite, which allows its pointing procedure to be calculated in a highly accurate way. Then, only the xe2x80x9cphysicalxe2x80x9d pointing inaccuracy remains (coupler and actuator related tracking error).
Compared with this terminal simplification, the navigator-controlled synchronization method according to the invention only requires few resources (software) in order to enable each terminal to calculate the satellite/terminal propagation delay.